st segregated more or less completely into some
cells, leaving the other cells without it. But in any case, after this
process of cell-division has proceeded for a certain time,
differentiation begins to set in--some cells become modified in one way,
others in another, and from what was a relatively homogeneous mass an
organized embryo, with highly differentiated parts, appears. The problem
immediately propounds itself--what are the factors which control this
differentiation? This problem is essentially a physiological one, and
yet, since it arises most conspicuously in a field which has been worked
by professed zoologists rather than physiologists, it has been studied
more by those trained in zoology and botany than by those who have
specialized in physiology. In this way, as in many other directions,
such as in the study of heredity, of sex, and of the effects of the
environment on the colours and structure of animals, the trend of
zoology in recent years has returned towards the physiological side, and
the old division which separated the sciences (but which has never so
seriously affected students of plant life) is being obliterated.

Hence we are led back to consider the progress of Physiology as a
whole--a subject with which the present writer hesitates to deal except
in a very superficial manner. Physiology as an organized science has
inevitably been deeply influenced by its close relation with medicine,
with the result that through a large portion of the period under review
it has concerned itself chiefly with the functions of the human body in
particular, or at least chiefly with Vertebrates from which, by analogy,
the human functions may be inferred. In this field it has made enormous
progress, and a vast amount of knowledge has been gained with regard to
the function and mechanism of all the parts and organs of the body. It
may perhaps be suggested, however, that in the pursuit of this detailed
(and in practice absolutely necessary) knowledge, physiologists have to
some extent lost sight of the wood in their preoccupation with the
trees. That is to say, while they have advanced an immense distance in
their knowledge of organs, they have not yet got as far as might be
hoped in the understanding of the organism--which is to say no more than
that the great and fundamental problem of Biology, the nature and
meaning of Life, is apparently almost as far from solution as ever. To
this further reference will be made b